Why are people not opening new campgrounds/RV parks?

All of the above are true. best thing is to find one that is already zoned and doesn't need anything more than some updating. cheaper off the bat and in the long run. you would just need to build the customer base back up, from what a previous owner possibly did...….WOW! this sounds easy, why didn't I think of this before?!!!

It's only a good investment if you can charge enough to people who are camping to cover the costs of running the campground and the capital expenses of building it, amoritized over some reasonable period, and still make a reasonable profit. That's very often an iffy proposition at best in practice between real estate costs, zoning and environmental regulations, payroll, etc. etc.; and frequently there are other businesses or uses for land that offer a better return with less risk for the investors.

Cost of land, environmental regulations, permits, water and sewer lines, electric service, road grading, landscaping, more permits, more regulations.
Probably more permits and regulations I haven't thought of. Plus on top of all that you are paying taxes before you even open the park.

Actually, if you want to make money you should build an RV storage lot. That's where most of those RVs are spending most of their time. Very few full-timers or part-timers relative to the number of RVs. Most are used just a few weeks a year.

Uh....it's expensive. Land in California? There's this REALLY NICE 98 acre parcel not far from me with it's own pristine lake. The homes around it? $1,000,000 and up. When I saw this parcel, the first thing I thought was "Perfect high end RV resort." But, by the time you did all the infrastructure, permitting, ect., ROI would be decades.